A conventional television receiver, such as a receiver according to NTSC broadcast standards adopted in the United States and elsewhere, displays a line interlaced image with a 4.times.3 aspect ratio (the ratio of the width to the height of the displayed image). Recently, significant interest has developed concerning the use of enhanced image display formats including wider image aspect ratios, e.g., 16.times.9 or 5.times.3, and progressively scanned images.
Two prominent types of compatible widescreen (wide aspect ratio) television signal processing systems are the "side panel" system and the "letterbox" system. In a side panel wide aspect ratio display system, left and right image side panels are spliced to a main image panel to produce a wide aspect ratio image with a 16.times.9 aspect ratio. One type of NTSC compatible enhanced definition widescreen system is described in U.S. Pat. No. 4,979,020--Isnardi. The letterbox system is commonly used in Europe for television broadcasting of wide aspect ratio movies. In such case a standard 4.times.3 aspect ratio television receiver receives and displays a vertically cropped image. The vertical cropping causes a displayed image to exhibit a wide aspect ratio such as 16.times.9, and results in black bars being produced along the top and bottom edges of the displayed image. Although the side panel widescreen format avoids the black bars of the letterbox format, signal processing associated with the side panel format is more complex.
Both the side panel system and the letterbox system may display an image in progressive scan (line sequential) format rather than interlaced scan format to produce what is perceived as a higher resolution display. Often this involves converting an interlaced image signal to a progressive scan image signal with the assistance of a so-called "helper" signal. For example, in the context of a compatible letterbox system, when the original (source) image information is in progressive scan format, a letterbox encoder subsamples the progressive scan image signal to create an NTSC compatible line interlaced signal. The encoder also generates a vertical helper signal prior to subsampling to help convert the compatible letterbox coded interlaced signal back to the original progressive line scan format at a wide aspect ratio progressive scan receiver. The helper signal may be transmitted in the bar regions of the compatible letterbox coded signal. At a wide aspect ratio receiver, the vertical helper signal recovered from the bar regions allows the wide aspect ratio receiver to recover vertical resolution that would otherwise be lost in conversion process from progressive scan to interlaced scan back to progressive scan. The helper signal also helps reduce vertical-temporal aliasing in moving portions of the image.
In order to prevent the bar regions of a wide aspect ratio letterbox image displayed by a standard aspect ratio receiver from distracting a viewer, i.e., to reduce the visibility of the helper signal, the helper signal may be attenuated and shifted in a black image direction at the encoder to assure that the bar region appears black when displayed, for most if not all images. At a wide aspect ratio receiver the helper signal is amplified in the amount of the attenuation factor before being utilized. Techniques for developing helper signals to facilitate the conversion of an original progressive scan signal to an interlaced signal and back to the original progressive scan form are well known. For example, one such technique in the context of a side panel widescreen television system is described in U.S. Pat. No. 4,979,020--Isnardi. Suzuki et al. disclose the use of a vertical-temporal helper signal for interlaced to progressive scan conversion in the context of a letterbox television system, in "Matix Conversion for Improvement of Vertical-Temporal Resolution in Letterbox Wide-Aspect TV", SMPTE Journal, February 1991.
It is herein recognized that transmission channel noise can degrade the helper signal sufficiently to adversely affect the quality of an image displayed at a receiver. This is particularly likely to occur, for example, when the helper signal and the main image signal are subjected to different types of signal processing. In the case of a letterbox system, it is recognized that channel noise is likely to adversely affect a helper signal that is companded in the bar regions of a compatible letterbox television signal. A system according to the present invention decreases the effects of noise in a decoded widescreen image due to noise corruption of the helper signal.